This invention pertains to polymer compositions curable by actinic radiation and more particularly to the use of mixtures of cycloaliphatic epoxides, polyols and onium salt photoinitiators for coating various metal, ceramic, glass, plastic, and composite substrates.
Prior art conventional conformal coatings comprise solutions of polymers or reactive components in a solvent. After application to a printed circuit board substrate, the solvent was removed by long time evaporation at room temperature or at an elevated temperature. When reactive components are involved, after mixture of two or more components, the systems had a relatively short pot life on the order of a few hours or less to about 24 hours.
Conformal coatings are coatings that encapsulate a printed circuit board and its components. This complete envelope of encapsulating material provides protection from humidity, moisture, dust, fungus, and other contaminants. In addition, properly applied conformal coatings enhance circuit reliability by eliminating current or voltage leakage from high impedance circuits. A paper by J. Waryold, of HumiSeal, entitled "How to Select a Conformal Coating for Printed Circuit Boards" describes conformal coatings. Section 4.7 of the book Protective Circuit Coatings describes the various types of materials that are used for conformal coatings.
A great deal of effort was expended on investigating the various cycloaliphatic epoxide materials as rapidly curable, ultra-violet light curable conformal coatings. Although curable coatings could be formed from many systems, they had electrical insulation values that only marginally passed the military specification requirements (i.e., they were about 2.5.times.10.sup.12 ohms). In addition, when subjected to a humid environment, the electrical insulation values decreased to a level less than the requirements of the military specification. The resistance values of the coatings could be improved if a thermal post cure were employed, but this is less desirable than coatings that do not require a thermal post cure or only require a thermal post cure to achieve extremely high performance features which exceed the military specifications by a large amount.
U.S. Pat. No. 4,256,828 discloses information about photocurable compositions that contain epoxides, organic material with hydroxyl functionality and a photosensitive aromatic sulfornium or iodonium salt.
U.S. Pat. No. 4,216,288, U.S. Pat. No. 4,193,799 and U.S. Pat. No. 4,108,747 are concerned with photocurable compositions that can be used in the electronics industry as photoresists.
However, none of the patents listed above deal with conformal coatings.
It is therefore an object of this invention to provide coating compositions that are moisture resistant and have electrical insulation resistance greater than about 2.5.times.10.sup.12 ohms when cured by exposure to actinic radiation, particularly ultra-violet light; and can be applied as conformal coatings to encapsulate or coat a printed circuit board and/or particular components of the printed circuit board.